Dynamic power supplies (DPS) are used in modern radio frequency (RF) transmitters, in particular, polar modulation transmitters and envelope tracking ET transmitters, to help increase energy efficiency. In the polar modulation transmitter 100 (see FIG. 1), the polar modulation transmitter's PA 102 comprises an energy-efficient nonlinear PA (for example, a Class-E or Class-F switch mode PA) that modulates a “signal envelope” carried by the DPS voltage VDD(t) produced by its DPS 106 onto the polar modulation transmitter's RF output RFOUT. The DPS voltage VDD(t) in the ET transmitter (see FIG. 2) also tracks the input signal envelope voltage Venv. However, unlike the polar modulation transmitter 100, the ET transmitter 100 employs a linear PA 202 (for example, a Class-A, B or AB linear PA), which does not operate as a modulator. Instead, the linear PA 202 operates as a controlled current source and the ET transmitter's 200's DPS voltage VDD(t) tracks the input signal envelope only for the purpose of maintaining the linear PA 202 just outside of saturation, where it operates linearly and near peak efficiency. In other words, while the polar modulation transmitter 100 achieves high energy efficiency by virtue of its energy-efficient nonlinear PA 102, the ET transmitter 200 achieves high energy efficiency in spite of its energy-inefficient linear PA 202 by continually forcing its linear PA 202 to operate at the boundary of saturation.
The accuracy of the signal envelope produced in the RF output RFOUT in the polar modulation transmitter 100 is highly dependent upon the precision of its DPS 106. On the other hand, because the DPS voltage VDD(t) produced by the DPS 206 in the ET transmitter 200 is not used to modulate the ET transmitter's RF output RFOUT, and need only control the DPS voltage VDD(t) so that the linear PA 202 does not saturate, the DPS 206 in the ET transmitter 200 does not have to be as precise as the DPS 106 in the polar modulation transmitter 100. Nevertheless, the more precise the DPS 206 is made to be, the more energy efficient the ET transmitter 200 becomes. Accordingly, whether used in a polar modulation transmitter or in an ET transmitter, it is desirable for the DPS to be both energy efficient and precise.
DPSs can be constructed from DC-DC converters, linear regulators, or a combination of both. DC-DC converters employ transistors that are controlled to operate as switches in order to achieve a high energy efficiency. However, they are not very precise and can produce a significant amount of switching noise and ripple at their outputs. The linear regulator avoids this problem by using an error amplifier and negative feedback to control load current flowing through a pass transistor disposed between the regulator's input and output. The pass transistor is not switched on and off, as are the transistors in DC-DC converters. Instead, the pass transistor is controlled to operate as a variable resistor. Unfortunately, this results in power dissipation and a reduction in energy efficiency, particularly when the magnitude of the linear regulator's output voltage (which is the DPS voltage VDD(t) when the linear regulator serves as a DPS) is significantly lower than its input DC voltage, which can occur frequently in an RF transmitter.
To overcome this problem but still enjoy the precision that the linear regulator offers, the linear regulator 304 can be connected in parallel with the DC-DC converter 302, as illustrated in FIG. 3. According to this approach, the DC-DC converter 302 operates as a current source, supplying most of the load current iOUT, and the linear regulator 304 serves as a voltage regulator, sinking and sourcing current to and from the DPS's output node 306, as necessary, to remove the switching noise and ripple that would otherwise be present in VDD(t) if only the DC-DC converter 302 was present. A further understanding of conventional DPSs and how they are constructed and operate may be found in the book: “Dynamic Power Supply Transmitters, Envelope Tracking, Direct Polar andHybrid Combinations,” The Cambridge RF and Microwave Series, First Edition, Cambridge University Press (2015), by Earl W. McCune.
One problem the parallel DC-DC converter/linear regulator DPS 300 has, however, is that whenever the linear regulator 304 is sinking current from the output node 306, that current is diverted away from the load (the PA 308), i.e., does not flow into the PA 308. Since the diverted current does not contribute to any useful work, the energy efficiency of the parallel DC-DC converter/linear regulator DPS 300 is therefore compromised. If the diverted current is large and frequent, the very reason for connecting the DC-DC converter 302 and linear regulator 304 in parallel in the first place can be defeated. The amount of diverted current can be ameliorated to some extent by increasing the switching frequency fSW of the DC-DC converter 302, which would then result in a lower ripple voltage. However, increasing the switching frequency fSW negatively impacts the DC-DC converter's 302's efficiency and undesirably reduces it output dynamic range.
Another limitation the parallel DC-DC converter/linear regulator DPS 300 has is that it is incapable of operating with high precision and high energy efficiency over a wide bandwidth, which is an important set of attributes for a DPS to have, particularly when the DPS is used in a polar modulation transmitter, given that the bandwidth of the envelope signal is often very wide and due to strict signal accuracy requirements often imposed by standards. In a telecommunications system application supporting the long term evolution (LTE) air interface, for example, which has channel bandwidths up to 20 MHz, the DPS should have a bandwidth several times that, in order for it to be capable of producing an accurate DPS voltage VDD(t) at its output. Unfortunately, designing a parallel DC-DC converter/linear regulator DPS that can operate with this degree of precision and with high energy efficiency over such a wide bandwidth is not easily realized. The present invention provides solutions to this problem.